With development of electronic technology, switch mode power supply is widely used in electronic devices per smaller, lighter and more efficient. There are many circuit topologies for switch mode power supply, such as buck, boost, half-bridge, flyback and so on. Taking flyback topology as an example, the switch mode power supply has the following functional modules: an energy input module, an energy coupled module, an energy output module, a feedback module and a controlling module. An alternative current (“AC”) voltage is input through the energy input module and transformed into a smooth direct current (“DC”) voltage, which is further transformed into a high frequency signal through periodically turning ON and turning OFF a switch based on a feedback signal. The high frequency signal is coupled by a transformer and a steady DC voltage is output through the output module.
Besides a normal operating mode, electronic devices can also operate under a reduced load or standby mode (generally referred to as “light load”). Under light load conditions, a load needs very low power output from the power supply. If the controlling module still uses an original frequency to drive the switch, loss on the switch becomes significant and efficiency becomes low. One traditional solution is decreasing the frequency to a lower level. However, the frequency will below 20 KHz with the load becomes lighter. Thus, the frequency is in an audible frequency range to cause noise issues. As a result, an improved control circuit for switch mode power supply to avoid audible noise and achieve high efficiency is needed.